1. Field of the Invention
The present invention relates to image display systems and more particularly to display controllers that interface between a source of image data and a display device.
2. Description of the Related Art
Display controllers provide an interface between a data source and a display unit. The data source can, for example, include a central processing unit (CPU) that provides overall control of a pocket PC, hand-held PC, mobile phone, etc. The data source can also be a digital camera such as a CMOS camera that is included in a camera-enabled cellular phone with image capture capability, for example. The display unit can be, for example, a liquid crystal display.
Normally, a display buffer, i.e., video RAM (VRAM) is included in the display controller and is used to transfer image data to the display device. Thus, the image data is written to the VRAM, and then read out from the VRAM.
Sometimes, it is desirable to display more than one image on the display panel at the same time. For example, a moving image might be displayed with a message box or some other image overlapping a portion of the moving image. Alternatively, the moving image might be placed in one region of the screen which is otherwise filled with ancillary information. This method of displaying two images on a single display device is often referred to as “picture-in-picture” (PIP). For purposes of this document, PIP will include a “bottom image” which is sometimes referred to as a main image and fills one portion of the screen of the display device, and a “top image,” sometimes referred to as a PIP image, which is the image displayed on another portion of the screen. Where the two images overlap, the top image will be displayed. Generally, the bottom image will fill the display screen, but this is not required. Although generally, the bottom image will display a video stream, it should be noted that either image can be a moving image and further that both the bottom and top images can be still images.
For graphics chips which contain VRAM, data defining the top image is typically stored along with the main bottom image data in the VRAM. The top image data in memory is typically updated by the host and is used to display brief messages.
A counter/timer circuit (CTC) in communication with a VRAM memory controller keeps track of where the top image will appear on the display. Appropriate control signals are provided to the memory controller to fetch data either associated with the bottom image or the top image. This data is then sent to a display interface which then provides the data to the display device.
Recently, RAM embedded liquid crystal displays (LCDs) have become available. These LCDs can store one frame of data, for example. An exemplary display controller utilizing RAM-integrated display devices is disclosed in a commonly-assigned, related application which is cited above and incorporated by reference. Instead of using VRAM, this previously-disclosed display controller provides a data pipeline and clock control that manages the data passing to the display. However, in the previously disclosed controller, there was no provision for displaying two images simultaneously on the display screen.